As well known in the art, a demand for a holographic storage medium capable of storing plenty of data has been increasing continuously. Therefore, recently, various types of holographic storage media have been developed to implement high storage capacity.
In a recording mode of a holographic ROM system as a typical system using a holographic storage medium, a modulated signal beam which has been modulated by digital data to be stored in the holographic storage medium and a reference beam are interfered to generate a holographic interference pattern therebetween, and such interference pattern is stored in the holographic storage medium (e.g., shaped as a disk) formed of, e.g., an optical refractive crystal. The optical refractive crystal is a material reacting differently according to an amplitude and a phase of the holographic interference pattern. In a reconstructing mode of the holographic ROM system, a reconstructing beam which is substantially equivalent to a complex conjugate of the reference beam used in the recording mode is illuminated on the holographic interference pattern stored in the holographic storage medium, and digital data are reconstructed by picking up a reconstructed beam formed by a diffraction of the reconstructing beam by the holographic interference pattern recorded in the holographic storage medium.
Then, in order to allow the digital data from the reconstructed beam generated by the holographic interference pattern to be reliable, an incidence angle of the reconstructing beam must be identical to that of the reference beam or different from that of the reference beam by 180 degrees (if the reconstructing beam is the complex conjugate of the reference beam). In particular, the incidence angle of the reconstructing beam on the holographic storage medium must be controlled highly precisely since the holographic storage medium has an angular selectivity.
However, the reconstructed beam cannot be precisely picked up from the holographic storage medium since the incidence angle of the reconstructing beam on the holographic storage medium is fluctuated by a tilt phenomenon due to a wobbling of the holographic storage medium as the disk-shaped holographic storage medium is rotated at a high speed.
Further, since any disk rotated at a high speed is subject to the tilt phenomenon due to the wobbling of the disk, in an optical ROM system using an optical storage medium such as a digital video disk (DVD) in which digital data are read by reflecting the reconstructing beam, the reflected beam obtained from the reconstructing beam cannot be precisely picked up by the tilt phenomenon due to the wobbling of the optical storage medium.